geunyeoneun bimireul jal jikineun pyeoniraseo jungyohan iyagido mitgo hal su isseo.

-는 편이다 means to tend to..., to be the type that..., or to be rather....

So 비밀을 잘 지키는 편이다 means:

• she tends to keep secrets well
• she’s the kind of person who keeps secrets well

It sounds softer and less absolute than just saying 비밀을 잘 지킨다, which would sound more direct and definite.

Here, 지키는 is the verb form used to modify the noun 편.

• 지키는 편 = the side/type that keeps
• 지킨 편 would sound like the side that has kept, which does not fit the usual pattern here
• 지켜는 편 is not correct grammar in this structure

So the standard expression is:

• V-는 편이다 for action verbs
  
  • 먹는 편이다
  • 운동하는 편이다
  • 비밀을 잘 지키는 편이다

편이라서 comes from:

• 편이다 = to be the type/side
• -이라서 = because it is

So 지키는 편이라서 means:

• because she tends to keep secrets well
• because she is the type to keep secrets well

Why -이라서? Because 편 is a noun, so it combines with 이다.

You may also see 편이어서, which is basically the same but a bit less contracted.

Yes, very normal.

• 비밀 = secret
• 지키다 = to keep, protect, observe

So 비밀을 지키다 literally and naturally means to keep a secret.

Adding 잘 gives:

• 비밀을 잘 지키다 = to be good at keeping secrets

This is much more natural in everyday Korean than trying to translate English word-for-word in some unusual way.

Here 도 adds the meaning of also, too, or very naturally in this sentence, even.

So 중요한 이야기도 suggests:

• important things too
• even important matters

The feeling is:

• not only ordinary things
• but even important things

So 도 adds emphasis.

Literally, 믿고 할 수 있어 is something like:

• I can do/tell it trusting her

In natural English, that becomes:

• I can tell her because I trust her
• I can confidently tell her
• I can trust her with even important things

Why -고?

-고 often connects actions or states in a simple sequence or association:

• trust her, and then do/say it
• do it while trusting her

So 믿고 here gives the feeling of with trust or trusting her.

Also, Korean often leaves out repeated words. The fuller idea is something like:

• 그녀를 믿고 중요한 이야기도 할 수 있어
• I can trust her and tell her even important things

Here, 하다 stands for something like 이야기하다 or 말하다, since that idea is already clear from 이야기.

The understood subject of 할 수 있어 is most naturally I.

So the sentence is really understood like this:

• As for her, since she tends to keep secrets well, (I) can trust her and tell her even important things.

This is very common in Korean. The sentence starts with 그녀는 to talk about her, but the final clause can have an omitted subject, usually understood from context.

So:

• 그녀는 = topic: as for her
• (나는) ... 할 수 있어 = implied subject: I can...

Korean often leaves out subjects when they are obvious.

Not necessarily. Although 이야기 can mean story, it often also means:

• something to talk about
• what one has to say
• matter
• issue
• topic

So 중요한 이야기 often means:

• an important matter
• something important
• important things to talk about

In this sentence, it does not have to mean a literal narrative or story.

있어 is the casual, non-polite form.

So the sentence is in a casual speech level. If you wanted to make it polite, you could say:

• 그녀는 비밀을 잘 지키는 편이라서 중요한 이야기도 믿고 할 수 있어요.

And a more formal style would be:

• 그녀는 비밀을 잘 지키는 편이라서 중요한 이야기도 믿고 할 수 있습니다.

So this is mainly about speech level, not a change in core meaning.

Grammatically yes, but in everyday spoken Korean, 그녀 can sound a bit literary, written, or translation-like.

In real conversation, Koreans often:

• omit the pronoun if the person is clear from context
• use the person’s name
• use a title or relationship term

So in natural speech, you might more often hear something like:

• 비밀을 잘 지키는 편이라서 중요한 이야기도 믿고 할 수 있어.

with she simply understood from context.

Textbooks often use 그녀 because it makes the sentence easier to understand for learners.

The difference is mostly nuance.

• 비밀을 잘 지킨다 = she keeps secrets well

  • more direct
  • sounds like a firm statement

• 비밀을 잘 지키는 편이다 = she tends to keep secrets well

  • softer
  • more like a general tendency or personality trait

So -는 편이다 often sounds more natural when talking about someone’s usual habits or character without sounding too absolute.